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Rudder offset or not?

Old 01-17-2004, 08:54 AM
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FlyingPilgrim
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Default Rudder offset or not?

I have not flown C/L for about 30 years, (flying R/C for the last few, though). I decided to teach my son how to fly, and built a Cadet w/ an .049. Every C/L plane I had ever seen, as a kid and up until now (including the Cadet) had right rudder built in to keep the lines taut. Since Then I began to build the Stunt Chimp, and was informed (on this forum) it did not have any rudder offset. Curious, I went to Brodak's website, and learned that they did not recommend rudder offset to keep lines tight, but adjusting the placement of the control lines at the wing tips to give the plane the correct attitude and right pull. It makes sense, but doesn't seem like it would be as effective. When did this method become popular (has it always been used, but I just never noticed?), and is it as effective at keeping the lines tight? Are there certain models that it works better on than others? It seems you would always want some right rudder, even if only a small amount, but perhaps I am wrong. Please share.

Thanks,
Pilgrim
Old 01-17-2004, 09:32 AM
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Elwyn
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Default RE: Rudder offset or not?

On a small plank wing .049 ship I'd still be tempted to put in a few degrees of offset. It probably won't make a noticeable difference on a small trainer either way. Since a little 1/2A ship will be knocked around more in the wind it might help in keeping the plane tight on the lines. I think on a big stunt ship there is the concern of rudder offset possibly causing some unwanted yaw during manuevers. You won't be worried about that with a little trainer.
Old 01-17-2004, 09:38 AM
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FlyingPilgrim
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Default RE: Rudder offset or not?

Elwyn,
Thanks for reply. I was planning on using some offset in the Stunt Chimp, just in case. Does 10 degrees sound like too much?

Pilgrim
Old 01-17-2004, 06:21 PM
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Jim Thomerson
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Default RE: Rudder offset or not?

I don't think you will need any offset. You could make the rudder adjuatable and see how it works.

Jim
Old 01-17-2004, 07:17 PM
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Elwyn
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Default RE: Rudder offset or not?

Jim T has been at this for a lot longer than I have so his advice is probably pretty good. I wouldn't go over 10 degrees if offset is used.
Old 01-17-2004, 07:53 PM
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FlyingPilgrim
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Default RE: Rudder offset or not?

Thank you both for your advice. Are there certain crafts where the offset is desireable, or is this just "old school" design? Just curious.

Pilgrim
Old 01-17-2004, 10:33 PM
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clscale-RCU
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Default RE: Rudder offset or not?

Look at the picture below....my 80" span camera airplane (carries a 35mm still camera) and is powered with a OS-90 four stroke and as you can see it has no rudder. My A-20 twin engine profile scale has ZERO rudder offset.

The line guide location and wing tip weight have the most effect, especially the adjustable line guide

Just put the rudder straight with no offset and get the line guide and wing tip weight right and you will be just fine.

The aerial picture was taken in flight at the Sepuldeva Basin flying site in Van Nuys, California. You can see the model airplane shadow and other models on the flying circle.

Fred Cronenwett
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Old 01-17-2004, 11:14 PM
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downunder
 
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Default RE: Rudder offset or not?

Generally speaking rudder offset serves no purpose in keeping the lines tight as there are better ways to do it such as leadout position which yaws the nose outwards. Where it can come in handy is if you happen to lose all line tension and then the model will try to turn back out to the end of the lines. A small excess of tip weight does the same thing by allowing the model to bank slightly outwards. With an old stunter of mine that I use just to have fun with, I've knocked the entire fin (with rudder offset) off it completely by flying a little too low inverted and I've never noticed any difference in line tension on subsequent flights.

With my competition stunter though I've got considerable side area towards the rear and the affect of this in strong winds is very noticeable. On the upwind side I can see the nose turn out which overcomes the wind trying to blow it in on the lines and on the downwind side the nose turns in slightly. So I'd suggest using a fair sized fin with maybe a few degrees of rudder offset (it can't hurt).
Old 01-18-2004, 12:42 AM
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Elwyn
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Default RE: Rudder offset or not?

I remember a crude plank wing 1/2A ship I had years ago that had problems with line tension at steep angles after the rudder had been knocked off. I think it was an old Sterling F-84 with a replacement wing. I'm sure the leadouts were far from the optimal locaton though. We didn't make anything adjustable back then.
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Old 01-18-2004, 04:03 AM
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Old Sourdough
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Default RE: Rudder offset or not?

Jim T. and the others are correct. The Stunt Chimp needs no rudder offset. It does need 1° to 3° out thrust to the engine, however. Most slack lines on trainer type aircraft occur at a time when rudder offset is the least effective, i.e., when the plane is moving at slow speed. Engine out thrust works the same at all speeds when the engine is running. A little extra tip weight will also help keep your lines tight.
Old 01-24-2004, 04:57 PM
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puttputt
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Default RE: Rudder offset or not?

I agree that leadout position is probably the most important variable, but wouldn't you then offset the rudder a bit just to ALLOW the yaw that the leadouts are trying obtain? Wouldn't want a straight rudder to fight the leadouts. And why do I keep hearing about the ineffectiveness of the rudder at slow speed? You've got the prop blast, well, blasting away. When I send an RC straight up until it almost stops, a blast of throttle and rudder makes her yaw pretty smart. Don
Old 01-24-2004, 09:48 PM
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Default RE: Rudder offset or not?

The CL designers of the 1940's weren't engineers. Most were adolescents at the time. Offset merely SEEMED useful. It never was, however. Somehow, this fallacious idea became so popular that following designers who were engineers included it because it was expected. Even after it was thoroughly debunked by Will Bill Netzeband and others, some modellers refused to believe the logic. Some continue to do so.

Rudder Offset is worthless.
Look at the tens of thousands of model flying wings built after 1961 without any rudders at all. No addition line tension problems for those. None. Believe it.


K.

*
[8D]
Old 01-24-2004, 10:11 PM
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Jim Thomerson
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Default RE: Rudder offset or not?

I've had a couple of airplanes where, because of the wing structure, it was not easy to make adjustable leadouts. On these airplanes I attempted to compensate by making the rudder adjustable. Having done this, I fiddled with the rudder a bit to see what it does. If you put in rudder offset you can get more line tension but the airplane will not fly as well. It is generally known that airplanes which you want to maneuver will fly best when you are looking right straight down the wing, i e, the airplane is flying tangent to the circle. Oddly enough, if you have the leadouts too far back, or put in too much rudder offset, and induce yaw, the airplane will loose tension when you fly high. If you make things adjustable, you can fiddle with them to make the airplane fly better, or fly worse, and learn a lot.

Jim
Old 01-24-2004, 10:12 PM
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puttputt
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Default RE: Rudder offset or not?

Right, no rudder is better than a straight rudder that fights the yaw that the leadouts are trying to introduce. But if you've got a scale ship with a rudder, wouldn't you want a bit of offset to keep the rudder from trying to straighten out the yaw? You could brute-force it with more leadout trail than really necessary, but that seems like an inelegant solution and would cause more drag as the two fight each other. I wonder if anyone has tried a floating rudder with just enough friction on the pivot to tell us where the rudder wants to be. Don
Old 01-24-2004, 10:33 PM
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puttputt
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Default RE: Rudder offset or not?

Jim, we posted simultaneously there, didn't want to look like I ignored you. I'm restoring an old Akro with a rudder I could break off and re-glue. Perhaps I should break it off and get her running tangent, then when I find the rudder position that keeps her tangent, I'll know I'm close. It certainly is odd that a noticeable amount of yaw screws things up overhead. I wonder if that tends to occur when the "fight" I mentioned slows down the ship? Do people use visual aids like a little white post on the inboard tip? Then a dot on the fuse that would line up when tangent. Don
Old 01-24-2004, 10:52 PM
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clscale-RCU
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Default RE: Rudder offset or not?

All of the CL scale models I have flown have had ZERO rudder offset, one even had a rudder that moved with very little force (hand pressure). I have flown very small and very large CL mocdels and in every case the two most important features were:

1) line guide location
2) CG location

some of my models do not have any wing tip weight, and all of them have ZERO rudder offset, and I can slow down, lower the flaps and throttle back and land under power without any problems.

One of my friends was flying his Beech Baron CL scale model and rudder broke off in flight, no affect. and as you can see from my camera plane (see post above) it has no rudder at all. When I try to explain to an RC guy who doesn't realize the model is a CL model that it flies great without a rudder boy do they get confused....they just can't figure it out.

Based upon the multiple CL scale models I have flown (18 lb and smaller) always always always have an adjustable leadout guide for the sake of line tension. When you start flying Cl scale models that weigh in excess of 10lbs the line tension can be excessive and the line guide is only thing that can lower the line tension to mangagable levels.

I put in wingtip weight about 1/2 of the time and my engine thrust line is zero-zero.

My best flying CL scale/carrier model was a 41" hawker sea fury built from my own plans with the following specs:

1) 1 ounces of wingtip weight
2) zero engine down thrust
3) zero engine outboard thrust
4) 2 degrees of wing angle of attack relative to engine thrust line
5) elevator has zero degree angle of attack
6) adjustable leadout guide

Oh...and by the way the bellcrank location does not matter! I have an article that found in a magazine that covers this for those interested....it states that the leadout guide location is determines the flyability of the model.

enough said....forget the rudder offset and always put in a adjustable leadout guide what ever it takes.

Careful on the elevator angle of attack. Some full size airplanes and even some kits have positive angle of attack on the elevator...ugh DEADLY on flyability.

Fred Cronenwett
CL scale and Carrier for 15 years
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Old 01-24-2004, 10:54 PM
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rjbob
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Default RE: Rudder offset or not?

Rudder offset DOES work! It is not, however, needed or wanted on all planes. Take flying wings, for instance...any rudder (offset or not) is pretty much ineffective as the moment arm is so short. Besides, any C/L airplane going 100+ mph is going to stay out on the lines regardless. What little offset the engine has on a combat wing is needed mostly on launch.

A rudder, fixed or not, is a control surface and as long as there is relative wind acting on it, it will make a difference. Engine offset, line placement, AND rudder offset should should work in harmony...not against each other.

Putt Putt is correct in as much as the effectiveness of the rudder at slow speed. When an engine in the tractor configuration is honkin' up front, the rudder is working. And when the lines are a little slack in a wing over, the line placement is not as important as engine and/or rudder offset. But if one is using long lines or heavier gauge lines, tip weight and line placement will play a more important part.

All the methods we have traditionally used to control line tension work. Some planes, however, fly better without having to resort to every single one.

The bottom line...do what works best for the particular plane your building/flying.

Cheers,
Bob Edison
Old 01-25-2004, 10:35 AM
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Jim Thomerson
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Default RE: Rudder offset or not?

If your lines go slack in the wingover, you need to realize that something is not quite right and fix it. You see pictures in the old magazines of flyers practically putting the handle on the ground to get tension in the wingover. If you watch today, you will see many flying the wingover with arm fully extended. Partially a difference in power available, partially a difference in how we trim our airplanes today.

Jim
Old 01-25-2004, 08:08 PM
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rjbob
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Default RE: Rudder offset or not?

[quote]ORIGINAL: Jim Thomerson

If your lines go slack in the wingover, you need to realize that something is not quite right and fix it.



That was actually the point I was trying to make, Jim. A fix for a particular problem could possobly be rudder offset...or maybe engine offset. Line tension problems are not always cured by lead-out position...although, admittedly, that IS the cure a lot of the time.

The thing is, rudder (offset or not) is not always needed or wanted on some planes, but when used correctly, it is effective.

Cheers,
Bob
Old 05-27-2022, 07:50 AM
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Default Tight lines ? Sure thing.!!

Stumbled into this thread and coincidently I just yesterday sifted through some old model aircraft magazines (see attachments). Sure a radical solution to tighten the lines..!! We have a CL revival in our club at the moment and I consider building a twin motor electric CL model "along these lines"......Cheers/





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